Swimming pools require a large amount of heat to maintain the water in the pool at a temperature that is comfortable for swimming. Many people utilize solar heating systems to provide at least a portion of the heat that is needed to keep the water temperature at a comfortable level throughout the seasons.
One solar heating system that has been very successful in capturing solar heat and converting that heat into warmed water comprises an array of heat exchange tubes made of a dark, thermoplastic material. The array is typically mounted on the roof of a building near the pool, and pool water is circulated through the array using the centrifugal pump that also pumps water through the pool filter. The dark color of the thermoplastic tubes assures high absorption of solar heat.
Because there is little solar energy per square foot of surface area, an array of heat exchange tubes must have a large surface area to capture useful amounts of heat. Thus, an array of heat exchange tubes is usually wide and long, stretching in some instances from the bottom of the roof on which it is mounted to the top of the roof, and the array usually has thousands of small diameter tubes arranged in parallel and through which the pool water flows. The large number of tubes provides a large surface area that absorbs solar energy.
In typical solar panels, water flows in parallel through a large number of tubes, which causes low flow velocity. Flow within each tube is consequently laminar and not turbulent. Laminar flow does not promote good heat exchange between the fluid and the tube wall, but it is desirable to maintain a low flow rate through the tubes rather than increasing the flow rate to provide turbulence. More energy is lost to friction when flow is turbulent instead of laminar, and consequently pumping costs increase as flow becomes turbulent. Thus, it is desirable to maintain laminar flow in order to keep pumping costs low.
It is also very desirable to increase the amount of solar energy transferred into pool water per unit area of exposed array area, so that the array can be made smaller or the dependence on other heat sources such as natural-gas heaters can be further reduced or eliminated. It is therefore one object of the invention to provide a thermoplastic heat exchange tube or array that has improved heat transfer to the fluid flowing through the tube or array. It is another object of the invention to increase the heat transfer while not increasing the pressure drop substantially through a tube or array. It is another object to increase the amount of heat transferred for a given flow rate through a tube or array. These and other objects and advantages are apparent from the discussion herein.